The cAMP content of cultured human fibroblasts is increased by incubation of cells with several effectors, e.g., bradykinin, PGE1, isoproterenol, and choleragen. Bradykinin presumably produces its effects by stimulation of prostaglandin synthesis; isoproterenol and PGE1 by interacting with specific receptors "coupled" to adenylate cyclase; choleragen by promoting the ADP-ribosylation of a regulatory component of the cyclase system. Cellular responsiveness to these effectors can be independently altered by incubation of cells with agents, such as glutamine, indomethacin, dexamethasone, and choleragen. In preliminary experiments, bradykinin was found to increase cGMP but not cAMP content of cultured pig aortic smooth muscle cells.